The present invention relates to a method of manufacture of sheets, strips and foils which are readily deformable and low in ear formation with high strength from aluminum alloys of the type Al-Si-Mg.
It is known that thin sheets of aluminum and of aluminum alloys of medium to high strength are often used in competition with or in combination with tin plate, for cans and can covers, in which connection the most frequent sheet thickness amounts to 0.3 to 0.2 mm and in the course of development is further reduced. This assumes of course that the deformation energy for rolling of the extremely thin sheets remains within economical bounds, and similarly that the durability and strength of the sheets are sufficient and can be used without waste, by good deep drawing properties, especially by fine grain and reliably slight ear formation.
It is furthermore known that these generally established requirements with respect to thin sheet for manufacture of cans have thus far been partly satisfied in various ways. Thus for example tin plate starts by possessing the good strength and deformation properties of iron; but the iron must be protected against corrosion by a layer of tin, which however is exposed at cut edges, and the high natural hardness of the iron requires, as a consequence of the powerful work hardening or the strongly increasing resistance to deformation on cold rolling of thin sheets, a significantly increasing deformation work or deformation energy. Similarly critically, the deformation energy costs increase in cold rolling of thin sheets also with the employment of naturally hard AlMg(Mn)-alloys e.g. for manufacture of can lids with up to 5% magnesium addition. Attempts are made, by numerous graduations of the alloy content, to achieve the always necessary minimum strength with predetermined final thickness more economically, e.g. by avoiding intermediate annealing, but then one almost totally gives up the deformability, or seeks partial solutions, in which concessions are unavoidable as regards strength and in particular also as regards deep drawing properties, particularly in the formation of ears, for example in the manufacture of half-hard can bodies up to 10% edge wastage by reason of ears.
It is known from German Pat. No. 1,184,968 to satisfy the requirements mentioned initially as regards thin can sheets more economically and comprehensively than with AlMg(Mn)-alloys by employment of hardenable aluminum alloys, e.g. of AlMgSi 0.5. There the strength is raised to the level of tin plate by combined cold age hardening and cold working hardening and partial hot age hardening, while the latter is coupled with the baking on of lacquer usual with can sheet, which itself raises the extension at breakage.
The "further important advantages" of the method, put forward in German Pat. No. 1,184,968, namely solution annealing and quenching with already at least twice and preferably so far as three to five times final thickness, and bright rolling of the surfaces with grey annealing skins arising from troublesome pot annealing, identify however an imperfect state of current technique at that time. With the annealing furnaces at present available, free choice was restricted of the optimum conditions for a consequential saving of deformation energy on rolling of extremely thin sheets, and similarly for a desired fine degree of grain without stretching defects and flow marks upon deep drawing, especially however for a minimum formation of ears. With employment of continuous strip furnaces developed in the meantime, the thereby attainable spontaneous highly annealed recrystallisation at about 500.degree. C. solution annealing temperature produces a significantly altered freer choice of optimum preparation requirements; however with AlMgSi 0.5 and other standardised AlMgSi alloys this invariably does not yet lead to sufficient satisfaction of the requirements, which have in the meantime risen further.
This is true particularly of the uniform sliding surface activity of the metal lattice and the consequently resulting minimal formation of ears, necessary for the total employment of the optimum strength and deformability of thin deep drawing sheets. For this purpose further conditions determined by structure are necessary.
Now the purpose of the invention is to achieve this result, with elimination of the defects of the hitherto known methods, by a suitable selection of the alloy composition, and for extreme cases by optimised working conditions for particular processing steps.